Mixer-based time domain reflectometer and method

ABSTRACT

An apparatus to measure optical characteristics of a fiber optic transmission line or other optical medium may include a source to generate a bipolar pulse signal waveform. The apparatus may also include a mixer to mix the bipolar pulse signal waveform and an optical pulse and reflected signal waveform from the fiber optic transmission line or other optical medium to form a mixed product waveform, wherein the reflected signal is responsive to the optical pulse being transmitted into the fiber optic transmission line or optical medium.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/277,239 filed on Mar. 23, 2006, now U.S. Pat. No. 7,667,830, which isa Continuation-in-Part of U.S. patent application Ser. No. 10/845,398filed on May 13, 2004, now U.S. Pat. No. 7,030,975, the disclosures ofwhich are hereby incorporated by reference herein for all purposes.

This invention was made with Government support under contractN00019-04-C-0005 awarded by the U.S. Navy. The Government has certainrights in this invention.

BACKGROUND OF THE INVENTION

The present invention relates to optical signal transmission systems orthe like and more particularly to a mixer-based time domainreflectometer and method for detecting any reflections, anomalies ordefects in a fiber optic transmission line or other optical medium.

Transmission lines are commonly employed to communicate signals betweenvarious portions of an electronic system. For example, coaxialtransmission lines, waveguides, and even parallel arrangements ofmetallic conductors are typically employed as transmission lines in suchsystems. Increasingly, fiber-optic transmission lines or other opticalmedia are being used instead of conventional metallic transmission linesto communicate signals in electronic systems due to the generally highernoise immunity and lower signal attenuation properties obtainable insuch lines. Additionally, fiber-optic transmission lines are generallythinner and lighter than metallic conductors of comparable capacity.

In systems employing fiber optic transmission lines or the like,difficulties may arise due to degradation of the line resulting fromphysical damage, aging, poorly matched and/or damaged connectors, or forother reasons. In practice, difficulties with transmission lines arefrequently difficult to detect and diagnose, particularly in electronicsystems where only a single terminal end of the transmission line may beaccessible. Although a number of different methods are available todetect and diagnose transmission line difficulties, one useful andcommonly employed method is time domain reflectometry. In time domainreflectometry, an optical pulse or pulses may be transmitted into afiber optic transmission line or medium. Any anomalies or defects mayresult in a reflected signal which may be detected by a time domainreflectometer. Such reflectometers are usually formed from standardcomponents as opposed to custom parts to keep costs reasonable. Thesestandard components, such as mixers or the like, may require appropriateinput or drive signals and modulation signals for optimum operation andability to effectively measure and analyze input pulses and reflectedwaveforms and mixed or modulated waveforms to detect any reflections atselected segments along a fiber optic transmission line or other opticalmedium.

BRIEF SUMMARY OF THE INVENTION

In accordance with an embodiment of the present invention, an apparatusto measure optical characteristics of a fiber optic transmission line orother optical medium may include a source to generate a bipolar pulsesignal waveform. The apparatus may also include a mixer to mix thebipolar pulse signal waveform and an optical pulse and reflected signalwaveform from the fiber optic transmission line or other optical mediumto form a mixed product waveform, wherein the reflected signal isresponsive to the optical pulse being transmitted into the fiber optictransmission line or optical medium.

In accordance with another embodiment of the present invention, anoptical system may include a fiber optic transmission line or opticalmedium. The system may also include a mixer-based optical time domainreflectometer with a bipolar local oscillator to measure opticalcharacteristics of the transmission line or optical medium.

In accordance with another embodiment of the present invention, anaerospace vehicle may include a fuselage and other components. Theaerospace vehicle may also include a fiber optic transmission line oroptical medium disposed in the fuselage, other components, or both. Theaerospace vehicle may further include a mixer-based optical time domainreflectometer with a bipolar local oscillator to measure opticalcharacteristics of the transmission line or optical medium.

In accordance with another embodiment of the present invention, a methodto measure optical characteristics from a selected segment of a fiberoptic transmission line or other optical medium divided into apredetermined number of segments may include determining an averagevalue or voltage of an optical pulse and reflected signal waveformwithout any modulation on a local oscillator. The method may alsoinclude applying a bipolar pulse on the local oscillator at a time delaycorresponding to the selected segment and mixing the bipolar pulse andthe optical pulse and reflected signal waveform to form a mixed productwaveform. The method may also include determining an average value orvoltage of the mixed product waveform.

Other aspects and features of the present invention, as defined solelyby the claims, will become apparent to those ordinarily skilled in theart upon review of the following non-limited detailed description of theinvention in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a block diagram of an exemplary optical system including amixer-based time domain reflectometer with a bipolar local oscillator inaccordance with an embodiment of the present invention.

FIGS. 2A and 2B (collectively FIG. 2) are a flow chart of an exemplarymethod to measure optical characteristics from a selected segment of afiber optic transmission line or other optical medium in accordance withan embodiment of the present invention.

FIG. 3 is a graph of exemplary waveforms to measure opticalcharacteristics of a fiber optic transmission line or other opticalmedium in accordance with an embodiment of the present invention.

FIG. 4 is an illustration of an exemplary aerospace vehicle including anoptical system and mixer-based time domain reflectometer in accordancewith an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of embodiments refers to theaccompanying drawings, which illustrate specific embodiments of theinvention. Other embodiments having different structures and operationsdo not depart from the scope of the present invention.

The present invention is described below with reference to flowchartillustrations and/or block diagrams of methods, apparatus (systems) andcomputer program products according to embodiments of the invention. Itwill be understood that each block of the flowchart illustrations and/orblock diagrams, and combinations of blocks in the flowchartillustrations and/or block diagrams, can be implemented by computerprogram instructions. These computer program instructions may beprovided to a processor of a general purpose computer, special purposecomputer, or other programmable data processing apparatus to produce amachine, such that the instructions, which execute via the processor ofthe computer or other programmable data processing apparatus, createmeans for implementing the functions/acts specified in the flowchartand/or block diagram block or blocks.

These computer program instructions may also be stored in acomputer-readable memory that can direct a computer or otherprogrammable data processing apparatus to function in a particularmanner, such that the instructions stored in the computer-readablememory produce an article of manufacture including instruction meanswhich implement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer orother programmable data processing apparatus to cause a series ofoperational steps to be performed on the computer or other programmableapparatus to produce a computer implemented process such that theinstructions which execute on the computer or other programmableapparatus provide steps for implementing the functions/acts specified inthe flowchart and/or block diagram block or blocks.

FIG. 1 is a block diagram of an exemplary optical system 100 including amixer-based time domain reflectometer 102 with a bipolar localoscillator 104 in accordance with an embodiment of the presentinvention. The bipolar local oscillator 104 may be part of the mixer 106as described in more detail below.

The optical system 100 may include a fiber optic transmission line 108or other optical medium to transmit optical energy in the form ofoptical signals. The fiber optic transmission line 108 may be formed bymultiple line portions 110 that may each be joined together by suitableoptical connectors 112 to minimize signal reflections. The reflectometer102 may measure optical characteristics of the line 108. Measuringoptical characteristics may include, but is not necessarily limited todetecting reflections that may be caused by anomalies in the line 108 oroptical medium, such as discontinuities, defects, degradation or thelike.

The system 100 may also include an optical signal source 114 to transmitoptical signals. The optical signal source 114 may be a laser signalsource or other optical source. The optical source 114 may emit one ormore relatively short-duration pulses of optical energy towards apartial mirror 118 in response to an input signal 120 or other stimulus.The partial mirror 118 may transmit at least a portion of the opticalpulse 116 into a terminal end 122 of the fiber optic transmission line108. The optical pulse may then propagate along the length of the line108. When the optical pulse 116 encounters an anomaly in the line 108 oroptical medium, optical energy or a reflected signal pulse 124 may bereflected back toward the terminal end 122 of the line 108 or medium.The reflected optical pulse 124 or signal is a function of thedifference in indices of refraction between the fiber material and theother material (usually air) at the break. The reflected optical pulseenergy 124 may be substantially reflected by the partial mirror 118 intoan optical receiver 126. The optical receiver 126 may also detect theoriginal optical signal pulse or energy 116 along with any reflectedsignals or reflected optical energy 124 responsive to the optical pulse116. The detected original optical pulse 116 and reflected opticalenergy 124 or signals may be converted to electrical signals by theoptical receiver 126 forming a waveform that may be transmitted to themixer 106. The mixer 106 may be a commercially available mixer ratherthan a custom component to maintain reasonable costs. For example, themixer 106 may be an Analog Devices AD8343 active mixer, available fromAnalog Devices, Inc. of Norwood, Mass., or a similar device.

The system 100 may also include a bipolar pulse generator 128 or similarsignal source to generate a bipolar pulse. The bipolar pulse may includea predetermined characteristic for proper or effective operation of themixer 106. Many active mixer devices, such as the Analog Devices AD8343,expect a local oscillator input of either about +1 or −1, that is, thedevice may operate most optimally or efficiently when eithersubstantially heavily turned on in the positive direction orsubstantially heavily turned on in the negative direction. Accordingly,a negative going bipolar pulse may serve to provide optimal or effectiveoperation of the reflectometer 102 as described in more detail herein.

The bipolar pulse generator 128 may generate the bipolar pulse or pulsewaveform in response to the input signal 120 or other stimulus that mayalso cause the optical energy or pulse 116 to be generated by theoptical source 114. The bipolar pulse may be delayed by a variable delaymodule 130. As described in more detail herein, the variable delaymodule 130 may delay the bipolar pulse of a bipolar pulse signalwaveform by a selected time duration corresponding to any reflection ofa signal or pulse from a selected segment of the fiber optictransmission line 108 to measure optical characteristics from theselected segment of the line 108.

The bipolar pulse signal waveform from the variable delay 130 may beapplied to the local oscillator 104. The bipolar pulse signal waveformand the optical pulse and reflected signal waveform from the opticalreceiver 126 may be mixed in the mixer 106 to form a mixed productwaveform. Expressed in other terms, the optical pulse and reflectedsignal waveform may be modulated in the mixer 106 by the bipolar pulsesignal waveform. An output of the mixer 106 may be coupled to anintegrator 132. The integrator 132 may time average the product signalsor mixer output signals to provide a time-averaged output.

The fiber optic transmission line 108 may be divided or segmented into apredetermined number of segments (N) or intervals for purposes ofanalysis and identifying a location of an anomaly. Because the variabletime delay module 130 performs a gating function, the reflected energysignals may be generated only from the segment of the line 108 or mediumthat corresponds to the selected time delay. The reflected energy signalwaveform 124 may then be time averaged by the integrator 132 over theselected time interval and successively repeated for each of thepredetermined number of segments (N) to generate an integrated value forthe reflected energy or signal waveform over all of the segments of theline 108 or medium.

The system 100 may also include a microcontroller 136 to facilitatedetermination of the optical characteristics of the line 108 or mediumor to detect reflections resulting from anomalies in the line 108. Themicrocontroller 136 may include an analog-to-digital converter (A/D) 138to receive the time averaged output signals or waveforms from theintegrator 132 and to convert the signals to a corresponding digitalsignal or waveform.

The microcontroller 136 may also include a microprocessor 140. Themicroprocessor 140 may perform various control functions and analysis ofthe waveforms as described in more detail herein. The microprocessor 140may be coupled to an output device or system 142. In one embodiment ofthe present invention, the system 142 may perform additional analysis ofthe waveforms or data generated by the microprocessor 140. In anotherembodiment of the present invention the device or system 142 may be adisplay or other output device that may present the waveforms and otherdata to a user for analysis or evaluation. In a further embodiment, theoutput device or system 142 may be a buffer or similar storage device tostore the data for access by other external systems (not shown).

The microcontroller 140 may also control operation of the variable delaymodule 130 to selectively delay the bipolar pulse signal to correspondto different segments along the line 108 or medium for measuring opticalcharacteristics or detecting any anomalies or defects along the line 108or medium.

In another embodiment of the present invention, the integrator 132 maybe a radio frequency (RF) power detection unit or the like. The powerdetection unit 132 may receive the waveforms from the mixer 106 andgenerate a DC voltage corresponding to the power level of the waveforms.Accordingly, the power detection unit 132 may provide a continuous andgenerally constant DC voltage corresponding to the power level of thesignals from the mixer 106 to the A/D converter 138, which may transferthe power to the microprocessor 140 in digital form. The power detectionunit 132 may include an Analog Devices AD8362 TRU-PWR Power Detector, orsimilar device.

FIGS. 2A and 2B (collectively FIG. 2) are a flow chart of an exemplarymethod 200 to measure optical characteristics or to detect any anomaliesfrom a selected segment of a fiber optic transmission line or otheroptical medium in accordance with an embodiment of the presentinvention. The method 200 may be embodied in the optical system 100 ofFIG. 1 or a similar system and may be performed thereby. In block 202,an optical pulse signal may be transmitted into an optical medium, suchas a fiber optic transmission line, similar to line 108 of FIG. 1, orother optical medium. The optical pulse signal may be a laser pulse orsimilar optical pulse.

In block 204, a predetermined constant positive or negative localoscillator (LO) signal may be generated. A local oscillator of a mixer,such as local oscillator 104 of mixer 106 of FIG. 1, or a similardevice, may operate optimally if driven either substantially positive ornegative hard enough so that a radio frequency (RF) input signal ismultiplied by either about a +1 or about a −1.

In block 206, the optical pulse (V_(P)) and the reflected signal may bemixed with the constant LO signal. A waveform may be generatedcontaining the optical pulse and reflected signals or pulses. Referringalso to FIG. 3, FIG. 3 is a graph 300 of exemplary waveforms 302-306 tomeasure optical characteristics of a fiber optic transmission line orother optical medium in accordance with an embodiment of the presentinvention. The waveforms 302-306 may be generated in the mixer-basedtime domain reflectometer 102 of FIG. 1 and may be representative of theoutput signals of the A/D converter 138. The waveform 302 in FIG. 3 isan example of a waveform containing a detected optical pulse (V_(P))transmitted into a fiber optic transmission line or medium and reflectedpulses (V₁-V₄) resulting from portions of the optical pulse (V_(P))energy being reflected by anomalies, such as connectors similar toconnectors 112 of FIG. 1 or the like.

In block 208, an average value or voltage (V_(cAL)) of the detectedoptical pulse (V_(P)) and reflected signal waveform for a fiber optictransmission line or medium segmented into a predetermined number ofsegments (N) or intervals may be determined. The average value orvoltage may be determined without any modulation or signal being appliedto a local oscillator of a mixer, such as mixer 106 (FIG. 1). Theaverage value or voltage may be represented by equation 1:

$\begin{matrix}{V_{CAL} = {V_{P} + {\sum\limits_{i = 1}^{N}V_{i}}}} & {{Eq}.\mspace{14mu} 1}\end{matrix}$

In block 210, a bipolar signal or waveform having predeterminedcharacteristics for proper or effective operation of a mixer associatedwith a reflectometer may be generated. As previously discussed, manyactive mixer devices, such as the Analog Devices AD8343, expect a localoscillator input of either about +1 or −1, that is, for optimumoperation the device is preferably either substantially heavily turnedon in the positive direction or substantially heavily turned on in thenegative direction. Accordingly, the bipolar signal or waveform mayinclude a negative-going pulse or the like for substantially optimal oreffective operation of the reflectometer mixer. Waveform 304 in FIG. 3is an example of a negative-going pulse in accordance with an embodimentof the present invention.

In block 212, the bipolar pulse of the bipolar signal or waveform may betime delayed by a selected duration corresponding to a selected segment(ith segment) or interval along the fiber optic line or medium tomeasure optical characteristics or reflections from the selectedsegment. The negative-going pulse may be thought of as a window that ismoveable along the fiber optic line or medium in response to theselected time delay to measure optical characteristics or detectanomalies at the location of the window corresponding to a selectedsegment of the line or medium.

In block 214, the detected original optical pulse and reflected signalwaveform may be mixed, multiplied or modulated, such as in mixer 106 ofFIG. 1, by the bipolar pulse waveform to form a mixed product waveform.In block 216, an average value or voltage of the output of the mixer ormixed product waveform (V_(DISP)) for the fiber optic transmission linesegmented in the predetermined number of segments (N) may be determined.The average value or voltage of the mixed product waveform (V_(DISP))may be represented by equation 2:

$\begin{matrix}{V_{DISP} = {V_{P} - V_{i} + {\sum\limits_{j \neq i}^{N}V_{j}}}} & {{Eq}.\mspace{14mu} 2}\end{matrix}$

The average value or voltage may be determined by means, such as theintegrator 132 or power detector unit in FIG. 1, and converted to adigital form, such as by A/D converter 138. Referring to FIG. 3, theoptical pulse and reflected signal waveform 302 may be mixed with thebipolar pulse waveform 306 to provide the mixed product waveform 306after integration, such as by integrator 132, and conversion to digitalform by A/D converter 138.

In block 218, a value or voltage (V_(i)) at the selected segment (ithsegment) may be determined. The value or voltage at the selected segmentmay include determining the difference between the average value orvoltage of the detected optical pulse signal and reflected signalwaveform (V_(CAL)) and the average value or voltage of the mixed productwaveform (V_(DISP)). Accordingly, the value or voltage at a selectedsegment may be represent by equation 3:

$\begin{matrix}{{Vi} = {\frac{1}{2}\left( {V_{CAL} - V_{DISP}} \right)}} & {{Eq}.\mspace{14mu} 3}\end{matrix}$

In another embodiment of the present invention, the difference(V_(CAL)−V_(DISP)) may be amplified by a predetermined factor. Undersome circumstances this may be deemed appropriate to take full advantageof the range of the A/D converter. The predetermined factor may be afunction of the predetermined number of segments (N) or may be more orless for practical purposes.

In practice, the bipolar pulse or local oscillator window pulse toselect a segment as described above may not always align exactly with areflection pulse from the selected segment of the fiber optictransmission line or other optical medium. In the event of suchnon-alignment, part of the reflection pulse value will in one segmentand the other part will be in an adjacent segment. Adjacent segmentswith significant values may be assumed to be associated and numericallycombined, although this assumption may slightly reduce the temporalresolution of the measurement.

In block 220, optical characteristics, such as any anomalies defects,discontinuities or the like, may be may be measured or detected in thefiber optic transmission line or other optical medium at the selectedsegment based on the value or voltage associated with the segment. Inblock 222, anomalies along the fiber optic transmission line or otheroptical medium may be detected by selectively delaying the bipolar pulseor window pulse to correspond to other segments along the line or mediumas previously discussed.

FIG. 4 is an illustration of an exemplary aerospace vehicle 400including an optical system 402 and mixer-based time domainreflectometer device 404 in accordance with an embodiment of the presentinvention. The aerospace vehicle 400 may be a commercial passengeraircraft as provide by the Boeing Company of Chicago, Ill. or other typeof aircraft. The optical systems 402 may be similar to the opticalsystem 100 of FIG. 1. Various embodiments of an optical system, similarto optical system 100 of FIG. 1 may be used in association with varioussystem and sub-systems of the aircraft 400, such as flight controlsystems, communications systems within the aircraft 400, such astelecommunications systems, in flight entertainment systems, Internetaccess systems and the like distributed to passenger seating, as well asother aircraft systems. The various embodiments of the optical system402 and reflectometer device 404 may be used to perform fault-checkingand/or operational monitoring of the fiber optic transmission lines orother optical medium that may be included in these various systems.

Although FIG. 4 illustrated the reflectometer devices 404 as possiblybeing an integral component of the aircraft 400, those skilled in theart will readily understand that one or more embodiments of thereflectometer device 404 may also be incorporated into a portable testdevice, such as device 406 that may be separately coupled to the varioussystems and sub-systems of the aircraft 400 to perform any ground-basedor other diagnostic analysis on selected optical systems.

The flowcharts and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems which perform the specified functions or acts, or combinationsof special purpose hardware and computer instructions.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Although specific embodiments have been illustrated and describedherein, those of ordinary skill in the art appreciate that anyarrangement which is calculated to achieve the same purpose may besubstituted for the specific embodiments shown and that the inventionhas other applications in other environments. This application isintended to cover any adaptations or variations of the presentinvention. The following claims are in no way intended to limit thescope of the invention to the specific embodiments described herein.

1. A method to measure optical characteristics from a selected segmentof a fiber optic transmission line or other optical medium divided intoa predetermined number of segments, the method comprising: determiningan average value or voltage of an optical pulse and reflected signalwaveform without any modulation on a local oscillator; applying abipolar pulse on the local oscillator at a time delay corresponding tothe selected segment of the fiber optic transmission line or otheroptical medium; mixing the bipolar pulse and the optical pulse andreflected signal waveform to form a mixed product waveform; anddetermining an average value or voltage of the mixed product waveform todetect an anomalies in the fiber optic transmission line or otheroptical medium.
 2. The method of claim 1, wherein applying the bipolarpulse comprises applying a negative-going bipolar pulse to the localoscillator.
 3. The method of claim 1, further comprising detecting anyreflections in the fiber optic transmission line or other optical mediumat the selected segment based on a value or voltage at the selectedsegment.
 4. The method of claim 1, further comprising determining avalue or voltage at the selected segment to detect any reflections atthe selected segment.
 5. The method of claim 4, wherein determining thevalue or voltage at the selected segment comprises determining adifference between the average value or voltage of the optical pulse andreflected signal waveform without any modulation and the average valueor voltage of the mixed product signal.
 6. The method of claim 5,further comprising amplifying the difference between the average valueor voltage of the optical pulse and reflected signal waveform withoutany modulation and the average value or voltage of the mixed productsignal by a predetermined factor.
 7. The method of claim 5, furthercomprising amplifying the difference between the average value orvoltage of the optical pulse and reflected signal waveform without anymodulation and the average value or voltage of the mixed product signalby a predetermined factor that is a function of the predetermined numberof segments.
 8. The method of claim 1, further comprising detecting anyreflections at any other segments along the fiber optic transmissionline or other optical medium by time delaying the bipolar pulse tocorrespond to any of the other segments.